A known airbag device includes an airbag and an airbag case that accommodates the airbag therein. The airbag device is mounted in the front passenger seat side of a vehicle in some cases. In these cases, the airbag device is mounted behind an instrument panel.
The instrument panel is provided with an opening for allowing the airbag to inflate therethrough into the vehicle interior. The opening is, in normal conditions, covered with an airbag lid. In a crash of the vehicle, the airbag inflates, pushes the airbag lid from behind, and thereby opens the opening.
The airbag device has an airbag lid manufactured integrally with the instrument panel these days, whereby the airbag lid is invisible from the outer surface side of the instrument panel. The instrument panel with this type of airbag device has a groove in the inner surface. The groove has a depth to the extent not to reach the outer surface. The groove surrounds an area, and the area defines the airbag lid. Note that the groove is referred to as “a tear line” or the like in some cases.
A known method of forming the groove is described in Japanese unexamined patent application publication No. 2004-82865. In the method, a heated blade is pushed against the inner surface of the instrument panel. With the method, the groove having the depth to the extent not to reach the outer surface of the instrument panel is formed in the inner surface of the instrument panel.
In another known method, laser light is emitted onto the inner surface of the instrument panel. Also with this method, the groove having the depth to the extent not to reach the outer surface of the instrument panel is formed in the inner surface of the instrument panel.
The known methods of forming the groove have problems as below.
With the method of forming the groove by use of the heated blade, the blade edge is heated with a heater or the like installed at the base of the blade. Therefore, heat conduction to the blade edge is not uniform, which causes unevenness in temperature at the blade edge. This results in a problem that it is difficult to form the groove having a uniform depth and width in the inner surface of the instrument panel.
Also with the known method of forming the groove, when the blade edge is heated, the blade edge is difficult to be cooled. This causes molten resin to be partially raised by the blade edge when the blade edge is pushed against the inner surface of the instrument panel and then pulled up. This results in the problem that it is difficult to form the groove having the uniform depth and width in the inner surface of the instrument panel.
As for the method of forming the groove by use of the laser light, since the laser light is equal to or more than 1 mm in width, the formed groove is unexpectedly wider. This causes a problem that the groove formed in the inner surface of the instrument panel can be seen through the outer surface of the instrument panel. That is, there is the problem that the groove formed in the inner surface of the instrument panel is unexpectedly visible from the outer surface side.